The present invention tackles the problem of measuring echo parameters on a telephone line.
The echo phenomenon, and in particular the echo to the speaker, can occur, for instance, when the telephone line connecting two users exceeds a certain length. The phenomenon causes a degradation of the communication quality, which degradation is less and less tolerable as the return time and the level of the echo increase. For this reason, over the years, the use of echo suppressors, particularly on satellite connections, has gradually become widespread, and eventually the international standardizing bodies have prescribed it. In order to operate correctly, those echo suppressors must know the characteristic parameters of the negative phenomenon to be contrasted and the suppressing action is all the more effective the more precise and accurate is the measurement of the characteristics of the echo.
For this purpose, measuring the round-trip propagation time and the residual level of a test signal injected into the line is generally known in the art.
The problem of identifying the most important information to be extracted from the received echo, i.e. the time interval between the echo and the test signal (or stimulation signal) sent onto the line, would be easily solved if the transmission channel were ideal, i.e. with unlimited band and without noise. Actually, in such case all that needs to be done is to determine the instant at which the received echo reaches its peak value with respect to the stimulation signal sent into the line. The availability of an unlimited band, or at least a very broad band, would also allow conferring to the test signal the nature of an impulsive signal, with very steep front, with the subsequent possibility of measuring with high precision the instant of arrival of an essentially identical return signal.
The need to operate on a telephone channel (thus a channel whose band is essentially limited from 300 to 3400 Hz) makes the measurement much more critical, also in consideration of the fact that the line is affected by noise, so that the simultaneous presence of echoes and of a high level of background noise often occurs.
A solution often adopted for measuring echo parameters on a telephone line is using, as a test signal, a sinusoidal signal which is injected into the line for a certain time interval and then stopped. The measurement of the echo is then compared to the measurement of the duration of the time interval elapsed between the instant which the test signal was stopped and the instant at which the corresponding return signal detected on the line ceases.
This solution is found to be unsatisfactory for several reasons.
First, the duration of the aforesaid time interval is determined not only by the echo phenomenon but also by the distortion (in the sense of a prolongation of the time duration, since a narrow band channel is dealt with) the test signal undergoes as an effect of its propagation along the line.
The test signal used in such solution can be seen as the product of a sinusoid by a rectangular window of a duration equal to the time interval between the instant wherein the test signal starts being injected into the line and the instant wherein that signal is interrupted.
The width of the window must preferably be a multiple of half the period of the sinusoid, and in phase therewith, in order to limit the harmonic content originated by the signal fronts. The response to such a stimulation signal by a real telephone channel comprises a train of sinusoids distorted by the effect of the channel, also in the form of a sort of offset (translation of the received signal with respect to an ideal vertical amplitude scale). This phenomenon, which can be ascribed to the cut off of the low frequencies, below about 300 Hz, typical of the telephone channel, makes it even more critical to detect correctly the transitions of the return signal. The aforesaid phenomenon can be recovered, but the related recovery time constant is too long with respect to the normal measurement time required to perform an effective intervention. In this respect it should be noted that the propagation delay (the quantity that in fact identities the echo phenomenon) typically does not exceed 30 milliseconds for a terrestrial telephone connection, and is about 260 milliseconds (in one direction, thus with a round-trip time of the signal of about 520 milliseconds), if the connection comprises a satellite link.
There is therefore a need to provide a solution that allows measuring in a precise and accurate manner the echo that may be present on a telephone line, taking into account the following requirements:
the test signal should occupy a limited band, compatible with the template of the telephone channel, in order to avoid the signal alteration phenomena mentioned above;
the test signal must have a limited time duration, in order to allow discrimination of echo times even with reduced duration without overlapping of the test signal and of the return signal;
an echo signal of acceptable amplitude must be obtained, with the consequent possibility of measuring its level (to be referred to the level of the test signal, which should preferably present a maximum level) so as clearly to discriminate the echo signal above the noise level.
The object of the present invention is to provide a solution capable of meeting the requirements mentioned above in an excellent manner, and in particular of meeting the first two requirements described, which in fact are mutually contrasting.
According to the present invention, this object is attained thanks to a method of measuring echo parameters on telephone lines which comprises the operation of injecting into the line (L) a test signal and detecting the corresponding return signal produced by the line (L) itself as an effect of the echo. The invention uses, as the test signal, an impulse (pulsed) sinusoidal signal with a lenticular envelope, with a limited time duration (a) such that a local maximum of the envelope can be identified both in the test signal and in the return signal, and having zero value at the beginning and at the end of its duration.
The test signal is generated starting from a first sinusoid with a first given frequency (xcfx89p). The duration (a) is linked to the period of the second sinusoid and can be equal to half the period of the second sinusoid.
The test signal (x(t)) can be expressed in the form:
x(t)=sin (xcfx89pt)xc2x7sin (xcfx89mt)xc2x7xcexc(t)xc2x7xcexc(ta),
where: xcfx89p is the first given frequency, xcfx89m is the second given frequency, and xcexc(t) is the unitary step function.
The return instant of the return signal is determined by detecting the rising edge of the envelope of the return signal. The return signal is subjected to full wave rectifying.
The test signal can be varied in an adaptive manner. The device for measuring echo parameters on telephone lines can comprise means for injecting on the line a test signal as well as means for detecting the corresponding return signal produced by the line itself as an effect of the echo. The device includes signal generating means arranged to generate, as test signal, an impulsive signal with lenticular envelope with a limited time duration (a) such that a local maximum of the envelope can be identified both in the test signal and in the return signal, and having zero value at the beginning and at the end of its duration.
The means for generating the test signal can comprise:
a first generator for generating a first sinusoid at a first given frequency (xcfx89p),
a second generator for generating a second sinusoid at a second given frequency (xcfx89m), lower than the first frequency, and modulating means for amplitude modulating the first sinusoid by the second sinusoid.
The means for generating the test signal can comprise means for generating a window signal, as well as means for applying the window signal to the test signal to confer it with a duration (a) related to the period of the second sinusoid.
The means for generating a window signal can comprise a divider element to confer to the window signal a duration (a) equal to half the period of the second sinusoid. The means for generating a window signal can comprise a control circuit to control the repetition rate of the test signal. Receiving means responsive to the return signal and arranged to detect the rising edge of the envelope of the return signal itself. The receiving means can comprise a rectifier circuit, preferably a full wave rectifier, acting on the return signal. The receiving means comprise filter means to reduce the noise content of the return signal. The filter means can comprise a first filter and a second tuneable filter in cascade. This filter means can be positioned upstream of the rectifier circuit.
The device can comprise a clipping circuit to transform the return signal into a square-wave signal. A low-pass filter can be interposed between the rectifier circuit and the clipping circuit.
A counter element can be started upon emission of the test signal and stopped upon detection of the return signal (R), the count value of the counter element at the stop being indicative of the return time of the echo to be measured. The counter element can be started in correspondence with the rising edge of the envelope of the test signal. The counter element is stopped in correspondence with the rising edge of the envelope of the return signal. Preferably the signal generating means are selectively adaptive to make selectively variable the test signal.